1. Field of the Invention
The present invention relates to a reflex type optical sheet sensor provided in a serial printer, and in particular, to an optically transparent dustproof plate provided in the reflex type optical sheet sensor.
2. Description of the Related Art
Recently, a serial printer, which has an another name of a dot matrix printer, is vigorously used with an electrical office equipment such as a word processor or a personal computer. The serial printer is for printing characters and/or images on paper. Generally, the serial printer uses two kinds of paper, a sheet of roll paper and a sheet of cut paper. The present invention relates to both and a word of "sheet" is used for the paper hereinafter. When the serial printer performs printing on a sheet, a foremost end and an endmost end of the sheet are required to be detected on a transfer way of the sheet in the serial printer. The detection of both ends, which will be called "sheet-ends" hereinafter, is for confirming whether the sheet is ready to be printed in the serial printer, and in particular, the detection of the foremost sheet-end is important for making the serial printer determine a start position of printing from the foremost sheet-end.
The sheet-ends can be detected by a mechanical switch such as a micro-switch. However, the mechanical switch causes mechanical trouble such as hurting the surface of the sheet or offering resistance against movement of the sheet transferred. Therefore, at the present time, an optical switch tends to be applied instead of the mechanical switch. A sheet sensor for the optical switch is called an optical sheet sensor. The sheet-ends are optically sensed by using light which is sent from a light source to a light sensor of the optical sheet sensor. When light is used, there are two ways of sensing the sheet-ends, one is a way of interrupting a beam of the light by the transferred sheet and the other is a way of using reflection of the light from the surface of the sheet transferred. Since the latter has an advantage of occupying a small space in the serial printer in comparison with the former, in other words, an advantage of making the serial printer compact, the latter is now used mostly. Since the present invention relates to the latter, the sheet sensor of the present invention will be called "reflex type optical sheet sensor" or simply "reflex type sheet sensor" hereinafter.
FIG. 1 is a perspective view of a typical serial printer 100 and FIG. 2 is a partial perspective view representing the operation of a reflex type sheet sensor 106 provided in serial printer 100 in FIG. 1. As shown in FIG. 1, serial printer 100 consists of a platen 101 having a cylindrical surface colored black, a main drive unit 102 for turning platen 101, a printing head 103 for performing printing on a sheet 200 (not depicted in FIG. 1 but depicted in FIG. 2) wound around the cylindrical surface of platen 101, a carriage 104 for carrying printing head 103 (mounted on carriage 104) so as to move printing head 103 in parallel with a rotating axis of platen 101, a carriage drive motor 105 for driving carriage 104 and reflex type sheet sensor 106 depicted in FIG. 2.
FIG. 2 shows reflex type sheet sensor 106 provided under platen 101. In FIG. 2, the same reference numeral as in FIG. 1 designates the same unit or part as in FIG. 1. When sheet 200 is fed into serial printer 100, sheet 200 is wound on the cylindrical surface of platen 101 by operating a sheet feed gear 108, passing threw a gap provided between the cylindrical surface of platen 101 and an inner cylindrical surface of a sheet cradle 109. The sheet 200 is held against the cylindrical surface of platen 101 tightly by a bail roller 108. Then, printing is performed on sheet 200 by the printing head 103 mounted on carriage 104. When sheet 200 is fed into the gap, sheet 200 is detected by a light beam 41 sent from reflex type sheet sensor 106. As shown in FIG. 2, a light pass hole 110 is provided to sheet cradle 109 so that light beam 41 goes therethrough. When there is no sheet 200 in the gap, light beam 41 is sent directly to the cylindrical surface of platen 101 through light pass hole 110 and absorbed by the blacked cylindrical surface. However, when sheet 200 is fed in the gap so as to cover light pass hole 110, light beam 41 is reflected by the surface of sheet 200, sending reflected light 42 back to reflex type sheet sensor 106 as shown in FIG. 2. Providing reflex type sheet sensor 106 thus, the sheet-ends of sheet 200 can be detected.
FIG. 3 is a partially cross sectional view of a reflex type sheet sensor 106' of the prior art, for showing that reflex type sheet sensor 106' performs sheet sensing by using light beam 41 emitted therefrom. In FIG. 3, the same reference numeral as in FIG. 2 designates the same part as in FIG. 2. As shown in FIG. 3, reflex type sheet sensor 106' consists of an optical device housing unit 1, an intermediate circuit board 3 attached to light device housing unit 1 and an interconnecting cable 7 connecting intermediate circuit board 3 to a main circuit board not depicted in FIG. 3, through a connector 6 attached to interconnecting cable 7. The light device housing unit 1 has concave structure in which a light source 4 and a light sensor 5 are provided on an inside wall of the concave structure. Since many sheets 200 are transferred through the narrow gap between platen 101 and sheet cradle 109 near by reflection type sheet sensor 106', a lot of paper dust are produced around reflection type sheet sensor 106'. Therefore, an aperture of the concave structure is covered by an optically transparent dustproof plate 2 for keeping light source 4 and light sensor 5 clean from dust. When sheet 200 appears at light pass hole 110, a ray of light beam 41 (see FIG. 2) emitted from light source 4 goes to the surface of sheet 200 through dustproof plate 2 and light pass hole 109 and reflected back to light sensor 5 through reflected light pass hole 109 and dustproof plate 2. The ray emitted from light source 4 and reflected back to light sensor 5 from sheet 200 is numbered as "ray 400" in FIG. 3.